Recently, an oxyfuel combustor has been studied as one of techniques for reducing emission of carbon dioxide (CO2) which is said to be one of factors for global warming; and attention has been attracted to, for example, a coal-fired boiler for oxyfuel combustion of pulverized coal. It has been conceived in such coal-fired boiler that oxygen is used as an oxidizing agent in lieu of air to produce combustion exhaust gas mainly composed of carbon dioxide (CO2) and such exhaust gas with high CO2 concentration is compressed, cooled and recovered as liquefied carbon dioxide for disposal. It has been conceived as one of ways of disposal that the liquefied carbon dioxide is stored in the ground. Such exhaust-gas treatment system for a coal-fired boiler for oxyfuel combustion is disclosed in Patent Literature 1.
As shown in Patent Literature 1, the exhaust gas from the coal-fired boiler for oxyfuel combustion of coal is known to contain not only carbon dioxide (CO2) but also impurities derived from coal feedstock such as nitrogen oxides (NOx), sulfur oxides (SOx), hydrargyrum (Hg), hydrogen chloride (HCl) and dust.
Among the above-mentioned impurities, sulfur oxides (SOx) may be contacted with and dissolved in water into sulfuric acid (H2SO4), and hydrogen chloride (HCl) may be dissolved in water into hydrochloric acid. Thus, such water-soluble sulfur oxides and hydrogen chloride may be disunited through contact with water.
Meanwhile, among the nitrogen oxides (NOx) as the above-mentioned impurities, nitrogen dioxide (NO2) may be contacted with and dissolved in water into nitric acid (HNO3); however, the exhaust gas from the coal-fired boiler has less oxygen (O2) so that nitrogen exists substantially in the form of nitrogen monoxide (NO) which is water-insoluble and thus is unremovable by water spraying or the like.
The above-mentioned sulfuric acid, hydrochloric acid and nitric acid are known to have a problem of eroding equipment in the exhaust gas treatment device; and hydrargyrum, which is trace metal, is known to hurt low-temperatured aluminum members constituting a heat exchanger. It is thus preferable that these impurities in the exhaust gas are removed at early stages. There is also a problem that admixture of the impurities into the exhaust gas lowers a degree of purity of the carbon dioxide, which makes troublesome the liquefaction through compression and cooling and requires increase in size of equipment. It is further feared that any sulfur oxides admixed to the carbon dioxide liquefied and stored in the ground may cause a reaction of the same with calcium in the ground, leading to failure in sealability of the underground storage. Thus, in a coal-fired boiler for oxyfuel combustion or other system where an exhaust gas mainly composed of carbon dioxide is produced and the carbon dioxide is disposed, it is extremely important to remove impurities in the exhaust gas.
Thus, as shown in Patent Literature 1, it has been conducted in the coal-fired boiler or the like for oxyfuel combustion that a spray-column- or packed-column-type or other wet desulfurizer used in a conventional air-fired boiler or the like is provided to remove sulfur oxides. Moreover, nitrogen and nitrogen oxides derived from coal feedstock are produced in the exhaust gas from the coal-fired boiler for oxyfuel combustion or the like, so that it has been conducted that a catalyst-type or other denitrator is arranged upstream of the desulfurizer to remove the nitrogen and nitrogen oxides.
It is known in the wet desulfurizer as mentioned in the above that water-soluble sulfur oxides and hydrogen chloride as well as dust contained in the exhaust gas are removed and that nitrogen oxides are partly removed and hydrargyrum, which is inherently low in content, is slightly removed. It has been conceived that if hydrargyrum in the exhaust gas is still high in concentration even after the above-mentioned exhaust gas treatment is conducted, a hydrargyrum-removing column is arranged to remove the hydrargyrum by adsorbent or the like.